Age-related (senile) cataract is principally a crystallin protein aggregation and subsequent precipitation disease that occurs over the time frame of several years. During cataract development, the increased sizes of aggregated and cross-linked crystallin multimers become so large that they finally become water insoluble and cause lens opacity. Deamidation and truncations of crystallins are identified as the most abundant post-translational modifications (PTMs), and are believed to be the major causative factors in age-related cataract development. However, the mechanisms of cataract development by either deamidated- or truncated-crystallins are still unknown. Based on our extensive results, we have hypothesized that deamidated and truncation of crystallins synergistically cause cataract by producing aggregates that deposit on lens membranes and lead to fiber cells degeneration. The aggregation process is initiated by truncated crystallin fragments that form amyloid fibril-type of complexes alone and/or with fragments of deamidated crystallins, filensin and phakinin. In this process, ?A3-protease-derived fragments of deamidated crystallins play a major role. We plan to test the above hypothesis by seeking answers to the following three questions: (1) What is molecular mechanism of cataractogenesis in ?A-N101D transgenic mouse model? Our first ever available transgenic mouse model with ?A-N101D transgene develops cortical cataract at about 7-month of age. Because several preceding phenotypic changes trigger cataract development at 7-months of age in transgenic mice relative to wild type mice, the mouse model will be used to elucidate the molecular mechanism of deamidated ?AN101D-induced cataract development. (2) How is the active site of ?A3 protease regulated and activated in vivo to proteolyze crystallins? We will determine how the active site ?A3 protease is regulated intrinsically by its N terminal arm and extrinsically by ?A- and ?B-crystallins as inhibitors, and how ?A3-protease preferentially proteolyzes deamidated and/or truncated crystallins relative to unmodified crystallins in vitro and in ?A-N101D transgenic mice. (3) What is the molecular mechanism of aggregation of fragments of crystallins (derived via proteolysis by ?A3-protease) and phakinin and filensin with a deamidated 4-kDa ?B fragment? It will be determined whether amyloidogenesis is the mechanism of aggregation among crystallin fragments (derived via proteolysis by ?A3-protease), fragments of filensin and phakinin, and an in vivo existing deamidated 4-kDa ?B fragment. Because mainly human lenses will be used in these studies, the findings will be relevant to in vivo mechanism of age-related cataract development. The results will be of significant therapeutic value to delay the development and progression of age-related cataract.